The present invention relates to industrial controllers used in controlling industrial machines and processes and, more particularly, to I/O modules being part of the industrial controller and providing an electrical interface between the industrial controller and the machine or process.
Industrial controllers are employed in industrial and commercial applications to control the operation of machines and processes. Generally the industrial controller executes a stored control program to control outputs to actuators on the machine or process according to inputs received from sensors on the machine or process.
Industrial controllers differ from conventional computers in providing real-time control subject to predetermined maximum delays. In addition, industrial controllers are normally constructed in a highly modular fashion to permit their hardware to be customized according to different control applications. In this latter regard, inputs and outputs to or from the industrial controller are normally handled by input/output (I/O) modules that may be attached to the industrial controller in different combinations to provide for the necessary electrical interface to the controlled machinery.
Different types of I/O modules may be used for different control applications. Input I/O modules provide specialized input circuits to receive signals from sensors and the like, and output I/O modules provide specialized output circuits to provide signals to actuators or the like. Two common output circuits are alternating current (AC) output circuits, typically employing an SCR or thyristor to switch an AC signal, and direct current (DC) output circuits typically employing a transistor to switch a DC signal. DC output circuits are often distinguished according to whether they provide “sinking outputs” that is, a switchable connection to ground that may receive current or “sourcing outputs” that provide a switchable connection to a power source that may output current.
In certain control applications, it may be desirable to provide both sinking and sourcing DC output circuits in a single I/O module. One method of accomplishing this, to be discussed further below, provides a single “floating” transistor that may be alternately connected to a load (e.g. an actuator) to provide a source of current or with a different connection, a sink of current.
Referring now to FIG. 1, a prior art universal output circuit 10 provides a first terminal 12 and a second terminal 14 connected across a drain and source terminal (respectively) of a field effect transistor (FET) 18.
The gate of the FET 18 is connected to a floating gate drive circuit providing a constant voltage 20 with respect to an isolated system ground 27 as received from a power supply 22. The output of the power supply 22 is floating with respect to an input system power source 24 and a system ground 25.
When the prior art universal output circuit 10 is operating in a sinking mode, an actuator 26 will have one terminal connected to terminal 12 and the other terminal connected to a positive terminal of an externally provided DC power source 28. The negative terminal of the externally provided DC power source 28 is connected to terminal 14. Activation of an optical isolator 30 by digital control signal 32 causes conduction of transistor 34 of the optical isolator 30 pulling down the source terminal of the FET 18 with respect to the gate terminal at voltage 20 to bias the FET 18 into conduction drawing current into terminal 12.
Referring now to FIG. 2, when the prior art universal output circuit 10 is operating in a sourcing mode, the actuator 26 must be disconnected from terminal 12 and connected to terminal 14. The remaining terminal of the actuator 26 is then reconnected to the negative power supply terminal of power source 28 while the positive power supply terminal of power source 28 is connected to terminal 12.
In this mode, activation of the optical isolator 30 by digital control signal 32 again pulls down the source terminal of the FET 18 with respect to the gate terminal, but this time to cause a sourcing of current through terminal 14. This dual mode of operation requires power supply 20 to be floating with respect to the power source 28. In addition, changing the mode of operation requires a rewiring of the actuator 26 with respect to the terminals 12 and 14 and the power source 28. This latter reconnection of terminals either requires changing the connections to the terminals of the I/O module or the use of an internal, high current capacity, multi-pole switch connected between the FET 18 and the terminals 12 and 14.
While this approach provides great flexibility in using outputs of the I/O module, the need to change connections to the single transistor requires undesirable switching circuitry or confusing change in external wiring procedures of actuators to the I/O module terminals. The control of a floating transistor requires a floating power supply that may be susceptible to damage.